Energy Implications of Retrofitting Retail Sector Rooftop

70 Pages·2012·2.03 MB·English

Energy Implications of
Retrofitting Retail Sector
Rooftop Units with Stepped-
Speed and Variable-Speed
Functionality
Daniel Studer, Rachel Romero,
Lesley Herrmann, and Kyle Benne

NREL is a national laboratory of the U.S. Department of Energy, Office of Energy
Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.
Technical Report
NREL/TP-5500-51102
April 2012
Contract No. DE-AC36-08GO28308
Energy Implications of
Retrofitting Retail Sector
Rooftop Units With Stepped-
Speed and Variable-Speed
Functionality
Daniel Studer, Rachel Romero,
Lesley Herrmann, Kyle Benne

Prepared under Task No. BEC7.1311
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy
Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.
National Renewable Energy Laboratory Technical Report
15013 Denver West Parkway NREL/TP-5500-51102
Golden, Colorado 80401
April 2012
303-275-3000 • www.nrel.gov
Contract No. DE-AC36-08GO28308
NOTICE
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Acknowledgments
The authors would like to thank the U.S. Department of Energy (DOE) Building Technologies
Program for its support. This report was prepared by the National Renewable Energy Laboratory
(NREL) Center for Electricity, Resources, and Building Systems Integration.
Many NREL colleagues provided valuable input and support. We are indebted to Eric Bonnema
for his model development assistance and contributions to the post-processing of simulation
results, to Ian Doebber for his insights on rooftop unit (RTU) design and fan flow controls, and
to Brent Griffith for his extensive EnergyPlus support. We also thank Matt Leach, who provided
information about energy management system controls for EnergyPlus and reviewed this report,
as well as Michael Deru, Feitau Kung, William Livingood, Paul Torcellini, and Ron Judkoff who
also reviewed this document.
This project would not have been possible without the contributions of the 2011 Retailer Energy
Alliance Heating, Ventilating, and Air-Conditioning (HVAC) Project Team. Thank you to the
project team members, particularly Scott Williams from Target, David Sheets from Walmart,
Paul Holliday from Lowe’s, and Kyle Wilkes from JCPenney. Their contributions improved the
representation of systems and controls.
iii
Executive Summary
Background
According to the 2003 Commercial Buildings Energy Consumption Survey (EIA 2008),
approximately 2.7 billion ft2 of commercial retail floor space is served by packaged air-
conditioning units. Often, these rooftop units (RTUs) are oversized to meet extreme and
infrequent thermal loads (Felts et al. 2000). Thus, RTU supply fans may provide air at rates that
are much higher than those needed to meet most thermal loads, wasting the energy needed to
move and heat or cool the excess air.
To address this issue, some U.S. retailers have started to upgrade existing RTUs, retrofitting their
constant-speed motors with stepped- or variable-speed functionality. Such retrofits may be more
cost effective than replacing an entire RTU, particularly if the unit is in the middle of its lifespan.
Other U.S. retailers, however, are uncomfortable pursuing this measure, as there is a lack of
supporting data detailing the climate zone-specific energy savings potential associated with this
upgrade. Building and portfolio energy managers have thus been unable to present a compelling
business case for RTU fan motor upgrades.
This study uses whole-building energy simulation to estimate the energy impact of stepped- and
variable-speed RTU fan motor retrofits in the retail environment, across 16 locations in all 15
U.S. climate zones. The results allow retailers to estimate the building-level energy savings
associated with this retrofit measure. This is a critical step in enabling retailers to determine
whether a compelling business case can be made.
Development Process
EnergyPlus Version 6.0 (DOE 2010) was used to evaluate the whole-building energy savings
associated with stepped- and variable-speed RTU fan motor retrofits as follows:
1. Two prototype big-box retail EnergyPlus models were created: one with refrigeration
systems and one without.
2. Each prototype model was replicated across 16 locations, which encompassed all 15 U.S.
climate zones, as defined by DOE (2005).
3. Standard 90.1-2004 (ASHRAE 2004a) was applied to the building envelope to create
climate-specific baseline models.
4. Custom sizing and control logic was used to modify a subset of the baseline models with
stepped- and variable-speed RTU fan motor controls.
5. EnergyPlus was used to simulate the energy performance of all models to determine the
energy savings associated with these retrofits.
iv
Results
Table ES–1 through Table ES–3 present an overview of the simulation results1. Annual whole-
building and electric energy savings were seen in all climate zones when switching to stepped- or
variable-speed fan motor operation. Whole-building energy savings ranged from 0.7%–8.4%.
Normalized annual electricity savings ranged from 0.50–1.67 kWh/ft2 (1.70–5.70 kBtu/ft2).
Average electricity savings across all locations were 1.17 kWh/ft2 (3.98 kBtu/ft2) for the stepped-
speed cases and 1.23 kWh/ft2 (4.40 kBtu/ft2) for the variable-speed cases. The results suggest
that for certain building types, annual whole-building electricity savings of 200,000 kWh may be
realized.
Heating energy increased in all climates, because less fan energy was imparted to the supply
airstream during the heating season. On average, this increase was 0.0149 therm/ft2 for the
stepped-speed cases and 0.0162 therm/ft2 for the variable-speed cases. (This increase was
always more than offset by the electricity use reduction.)
Based on the average energy savings, if variable-speed retrofits were implemented across 10% of
the 2.7 billion ft2 of retail space served by packaged RTUs (EIA 2008), retailers would save
approximately 332 GWh/yr (1.13 × 1012 Btu/yr) of electricity and increase natural gas use by 4.4
million therm/yr (4.34 × 1011 Btu/yr). Assuming typical utility rates of $0.10/kWh and
$1.00/therm, this would equate to annual utility cost savings of $28.8 million.
This measure requires that a relatively small number of RTU fan motors be upgraded per
building. Therefore, this report makes a case that such retrofits should be investigated in all
climate zones to address the oftentimes dispersed energy loads in commercial buildings.
The energy savings values presented here depend on a number of assumptions, including the
cooling coil and fan flow rate sizing methodology, sequence of operations, space set points, and
hours of operation. In order to provide broadly applicable findings, values were used that reflect
common practice. Appendix C provides instructions about how to use the results of this study to
estimate high-level utility cost savings. A retailer can use such results to justify site-specific
assessments of retrofit feasibility and determine if a compelling business case can be made.

1 Note that additional differences between the models with and without refrigeration, such as zoning layout and the
fan flow rate sizing methodology, affect the energy consumption and savings values. Readers should exercise
caution when comparing results between model types.
v
Table ES–1 Annual Whole-Building Energy Savings for All Models (%)
Models With Refrigeration Models Without Refrigeration
Climate
Zone Location Stepped-Speed Variable-Speed Stepped-Speed Variable-Speed
1A Miami, FL 2.5 3.5 2.3 2.3
2A Houston, TX 3.2 4.0 3.6 3.6
2B Phoenix, AZ 3.5 4.7 4.1 5.2
3A Atlanta, GA 3.2 3.9 3.7 3.7
3B Los Angeles, CA 6.1 6.5 8.1 8.3
3B Las Vegas, NV 3.4 4.6 4.1 5.5
3C San Francisco, CA 5.8 5.9 8.3 8.4
4A Baltimore, MD 2.2 2.6 2.5 2.4
4B Albuquerque, NM 3.1 3.8 3.6 4.1
4C Seattle, WA 3.1 3.2 4.0 4.0
5A Chicago, IL 1.8 2.1 2.1 2.0
5B Boulder, CO 2.6 3.1 3.2 3.5
6A Minneapolis, MN 1.5 1.8 1.7 1.7
6B Helena, MT 1.8 2.1 2.3 2.4
7 Duluth, MN 1.1 1.2 1.4 1.3
8 Fairbanks, AK 0.7 0.7 0.9 0.9

Table ES–2 Annual Electricity Savings for All Models (kWh/ft2)
Models With Refrigeration Models Without Refrigeration
Climate
Zone Location Stepped-Speed Variable-Speed Stepped-Speed Variable-Speed
1A Miami, FL 0.66 0.94 0.50 0.51
2A Houston, TX 0.95 1.13 0.89 0.91
2B Phoenix, AZ 1.00 1.30 0.98 1.22
3A Atlanta, GA 1.14 1.30 1.11 1.13
3B Los Angeles, CA 1.31 1.40 1.38 1.41
3B Las Vegas, NV 1.11 1.39 1.13 1.41
3C San Francisco, CA 1.47 1.50 1.66 1.67
4A Baltimore, MD 1.19 1.31 1.16 1.24
4B Albuquerque, NM 1.24 1.43 1.23 1.40
4C Seattle, WA 1.41 1.44 1.55 1.57
5A Chicago, IL 1.22 1.32 1.14 1.27
5B Boulder, CO 1.30 1.42 1.29 1.44
6A Minneapolis, MN 1.18 1.28 1.06 1.20
6B Helena, MT 1.30 1.39 1.27 1.43
7 Duluth, MN 1.24 1.29 1.13 1.29
8 Fairbanks, AK 1.12 1.18 1.00 1.14

vi
Table ES–3 Annual Natural Gas Savings* for All Models (therm/ft2)
Models With Refrigeration Models Without Refrigeration
Climate
Zone Location Stepped-Speed Variable-Speed Stepped-Speed Variable-Speed
1A Miami, FL 0.000 0.000 0.000 0.000
2A Houston, TX –0.005 –0.005 –0.005 –0.005
2B Phoenix, AZ –0.004 –0.004 –0.005 –0.005
3A Atlanta, GA –0.012 –0.012 –0.012 –0.013
3B Los Angeles, CA –0.002 –0.002 –0.002 –0.002
3B Las Vegas, NV –0.009 –0.009 –0.010 –0.010
3C San Francisco, CA –0.009 –0.010 –0.011 –0.011
4A Baltimore, MD –0.020 –0.020 –0.020 –0.023
4B Albuquerque, NM –0.017 –0.017 –0.017 –0.019
4C Seattle, WA –0.023 –0.023 –0.025 –0.026
5A Chicago, IL –0.023 –0.024 –0.020 –0.025
5B Boulder, CO –0.021 –0.021 –0.019 –0.023
6A Minneapolis, MN –0.023 –0.023 –0.018 –0.023
6B Helena, MT –0.026 –0.026 –0.023 –0.027
7 Duluth, MN –0.029 –0.029 –0.023 –0.029
8 Fairbanks, AK –0.026 –0.028 –0.021 –0.025
*Negative values indicate that natural gas use increased.

vii
Nomenclature
ACH air changes per hour
ASHRAE American Society of Heating, Refrigerating and Air-Conditioning
Engineers
c.i. continuous insulation
cfm cubic feet per minute
COP coefficient of performance
DOE U.S. Department of Energy
EPD equipment power density
EUI energy use intensity
ft foot, feet
ft2 square foot, square feet
HVAC heating, ventilating, and air conditioning
in. inch, inches
kBtu kilo-Btu
kW kilowatt, kilowatts
kWh kilowatt-hour, kilowatt-hours
LPD lighting power density
m meter, meters
m2 square meter, square meters
MJ megajoule, megajoules
NREL National Renewable Energy Laboratory
OA outside air
RH relative humidity
RTU rooftop unit
SAT supply air temperature
therm unit of heat energy equal to 100 kBtu
VAV variable air volume
w.c. water column

viii
Contents
Acknowledgments.......................................................................................................................... iii
Executive Summary ....................................................................................................................... iv
Background ................................................................................................................................ iv
Development Process ................................................................................................................. iv
Results ......................................................................................................................................... v
Nomenclature ............................................................................................................................... viii
Figures and Tables ......................................................................................................................... xi
Figures ........................................................................................................................................ xi
Tables ......................................................................................................................................... xi
1.0 Introduction .......................................................................................................................... 1
1.1 Objective .......................................................................................................................... 1
1.2 Scope ................................................................................................................................ 1
2.0 Building Energy Modeling Methodology ............................................................................ 2
2.1 EnergyPlus ....................................................................................................................... 2
2.2 Climate Zones .................................................................................................................. 2
2.3 Modeling Process ............................................................................................................. 3
3.0 Model Development............................................................................................................. 4
3.1 Form ................................................................................................................................. 4
3.2 Fabric ................................................................................................................................ 6
3.2.1 Opaque Envelope .......................................................................................................... 6
3.2.2 Vertical Fenestration ..................................................................................................... 7
3.3 Infiltration......................................................................................................................... 7
3.4 Internal Load Densities .................................................................................................... 8
3.4.1 Occupancy .................................................................................................................... 8
3.4.2 Plug and Process Loads ................................................................................................ 9
3.4.3 Refrigeration Loads .................................................................................................... 10
3.5 Schedules ........................................................................................................................ 10
3.6 Lighting .......................................................................................................................... 10
3.7 Heating, Ventilating, and Air Conditioning ................................................................... 11
3.7.1 System Type ............................................................................................................... 11
3.7.2 Rooftop Unit Performance Properties ........................................................................ 11
ix